Demands for diesel engines have continuously raised as the diesel engines are variously used in entire industries such as an automobile, shipbuilding, general industry and etc., and are possible of operation in high power and high load on the basis of high fuel efficiency and high reliability. In addition, as an employment of a diesel engine becomes obvious in a 3 L car program or super car project promoted for the purpose of a high fuel efficiency of an automobile, they are expected to increase automobiles with a diesel engine. However, the diesel automobiles are responsible for 40 percent of total air pollution, thus may be recognized as a principal offender of air pollution in developed countries.
To deal with the above, each country has reinforced a regulation of exhaust gases of the diesel automobiles. Such air pollution due to the diesel automobile is mainly generated by Nitrogen Oxides (NOx) and Particulate Matters (PM). Thus, main objective materials of the regulation of the exhaust gases of the diesel automobiles are the Nitrogen Oxides and the particulate matters, technologies for dealing with them are concentrated in decrease of a concentration of the Nitrogen Oxides by delaying of fuel injection timing and Exhaust Gas Recirculation (EGR) and decrease of the particulate matters by improvement and reformation of combustion performance of the engine.
As a method for reducing the Nitrogen Oxides, it is used a Selective Catalytic Reduction (SCR) which reduces Nitrogen Oxides to nitrogen and oxygen using a reducing agent on a catalyst.
Ammonia, urea and hydrocarbon may be used as the reducing agent, but the hydrocarbon is preferred because the ammonia and the urea have a disadvantage of requiring an additional supply infra, and types of the hydrocarbon are diesel, kerosene, propylene, propane, ethylene, butylene, methane and etc.
Meanwhile, a Diesel Particulate Filter is widely used for removing the PM, and regeneration methods can be classified as a passive regeneration type and an active regeneration type. In the passive regeneration type the Diesel Particulate Filter is regenerated by converting NO to NO2 over an oxidation catalyst on the DPF and produced NO2 oxidize the PM, and in the active regeneration type it is used an auxiliary device forcibly increasable a temperature of the filter for more active regeneration. In the active regeneration type may be an increase of the temperature of filter by using an electric heater, a usage of a plasma reactor provided in front of the DPF, an injection and burning of diesel fuel and etc.
As a conventional exhaust gas purifying device for a diesel engine is known an exhaust gas purifying device, wherein DPF on which the oxidation catalyst is supported is disposed at upstream of an exhaust passage, a hydrocarbon selective reduction type Nitrogen Oxide reduction catalyst (DeNOx catalyst) is arranged at downstream of an exhaust passage and a fuel injector is mounted between the DPF and the DeNOx catalyst.
However, the conventional exhaust gas purifying device is hardly successful in removing efficiently together the Nitrogen Oxides and CO and total hydrocarbon (THC) including the Particulate Matters in a middle temperature range.